Cyclic Bending Loads Research Articles

Rotary Fatigue Testing to Determine the Fatigue Life of NiTi alloy Wires: An Experimental and Numerical Analisys

Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (Memry™ and Euroflex™). The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.

The Stress Detection of Stainless Steel AISI 304, AISI 316L and AISI 316Ti During Three-point Bending Cyclic Loading

The article will describe investigation of the elastic and the plastic deformation of stainless steel during three-point bending cyclic loading with using thermovision. The analysis will prove different temperature response to external loading and dependence of elastic or plastic deformation development on material's state. The input data which are necessary for this analysis we will can get from temperature field of specimen surface. Process of elastic and plastic deformation is in dependence on radiation emitted by the object. For obtain thermal fields we will use thermal camera FLIR SC7000 with the highest sensitivity. For comparison, we will make also FEM analysis of three-point bending for this material. Stainless steel is resistance to corrosion and staining, low maintenance and familiar lustre make it an ideal material for many applications. There are over 150 grades of stainless steel, of which maybe fifteen are most commonly used. For our analysis we will use three of them: AISI 304 (DIN 1.4301), AISI 316L (DIN 1.4404) and AISI 316Ti (DIN 1.4571).

Surface crack growth subject to bending and biaxial tension-compression

Fatigue surface crack growth and the in-plane and out-of-plane constraint effects are studied through experiments and computations for aluminium alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading and bending central notched specimens with external semi-elliptical surface crack. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth rate and surface crack paths behaviour is studied under cyclic pure bending and biaxial tension-compression fatigue loading. This work is centered on the relations between crack size on the free surface of specimen considered configurations, COD and aspect ratio under different fatigue loading conditions. For the experimental surface crack paths in tested specimens the T-stress, the local triaxiality parameter h, the out-of-plane TZ factor and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface of the tested bending and cruciform specimens as well as the deepest point of the semi-elliptical surface crack front. As result fatigue surface crack paths or crack front positions as a function of accumulated number of cycle of loading are obtained.

Concrete-Filled Steel and Steel Tubular T-Joints under Cyclic in-Plane Bending

A series of tests on the concrete-filled and steel tubular T-joints under in-plane bending cyclic loading are presented in this paper. Two steel tubes were welded to form T-joint specimens, and then the chord was filled with self-compact concrete. For each test specimen the chord was loaded in a constant compression while the brace was under quasi-static cyclic in-plane bending loading. In total three concrete filled steel T-joints and two pure steel T-joints were tested. The failure modes, hysteretic curves and ultimate strengths of test specimens were obtained. The failure modes of all specimens were punching shear failure of the chord. It was shown that the filled concrete greatly enhanced the stiffness, the yielding strength and the ultimate strengths of T-joints. The energy dissipation ratio of the specimens under cyclic loading was calculated. The energy dissipation ability of concrete-filled specimens was better than that of steel specimens. A method provided by ECCS was used to calculate the yield strength of the test specimens. The ultimate strengths predicted by current AISC Standard and API RP2A specifications were compared with the test results.

Study on stiffness and recoverability of Fiber Reinforced Polymers-Reinforced Concrete (FRP-RC) composite T-beams with prefabricated Basalt fiber-reinforced polymers shell

This paper focuses on the stiffness and recoverability of a new type of composite beam, the Fiber Reinforced Polymers-Reinforced Concrete (FRP-RC) beam, in which steel bars are the main reinforcement. Ten pieces of beams under both static and cyclic bending load were conducted. Fiber-reinforced polymer profile made by the vacuum infusion molding process is used as reinforcement in the composite beams, in which basalt fiber-reinforced polymer sheets were axially laid at the bottom of the beams in order to improve bearing capacity, and hoop-directionally laid at the lateral side of the beams with the advantages of improving the shear resistance. A finite element model is proposed to predict nonlinear deflections and stresses. The flexural stiffness and recoverability index of this new type of composite beam are studied. The results show that the recoverability of the FRP-RC composite beam is better than that of the normal RC beam and it increases along with the fiber-reinforced polymers reinforcement ratio. Stiffness calculation formulas at various stages and residual deformation formulas are proposed. Finally, the calculation results and test results are compared, which shows that the formulas for stiffness and deformation demonstrate good precision.

On the response of dented stainless-steel pipes subject to cyclic bending moments and its prediction

In this paper, the effect of initial dent depth on the response of pipes subjected to cyclic bending loads with relatively large magnitudes is investigated experimentally. The initiation and propagation of the resulting cracks in the damaged region of the pipe are found to cause fatigue failure, augmented by the progressive accumulation of the cross-section ovalization during the cyclic loading. The empirical formula proposed by Kyriakides and Shaw [1] is modified based on the experimental results, in order to predict the number of cycles causing the local instability of pipe's cross-section. Moreover, it is shown that the experimental curve of the ovalization and the number of cycles necessary to produce buckling can be divided into three stages: initial, secondary and tertiary stages. Finally, an empirical equation, similar to the Bailey–Norton creep relationship, is proposed for simulating the initial and secondary stages of the aforementioned curve. The results produced by the proposed empirical relationship are found to be in good agreement with the experimental data.

Development of a bending test procedure for the characterization of flexible ECoG electrode arrays

Abstract An automatized mechanical test has been developed to characterize flexible ECoG and grid electrode arrays under cyclic bending load. Electrodes with different test structures were designed and the bending cycle dependant failure rate was found to follow a Weibull distribution. A corresponding failure mode analysis revealed a characteristic lifetime of more than 25000 bending cycles for some designs. However, design parameters as e.g. a smaller track width reduced this lifetime by a factor of two. In comparison to a similar BMI grid electrode array which was implanted for several months in an animal, failure patterns were comparable to those simulated with the bending test.

The ratchet–shakedown diagram for a thin pressurised pipe subject to additional axial load and cyclic secondary global bending

The ratchet and shakedown boundaries are derived analytically for a thin cylinder composed of elastic-perfectly plastic Tresca material subject to constant internal pressure with capped ends, plus an additional constant axial load, F, and a cycling secondary global bending load. The analytic solution is in good agreement with solutions found using the linear matching method. When F is tensile, ratcheting can occur for sufficiently large cyclic bending loads in which the pipe gets longer and thinner but its diameter remains the same. When F is compressive, ratcheting can occur in which the pipe diameter increases and the pipe gets shorter, but its wall thickness remains the same. When subject to internal pressure and cyclic bending alone (F = 0), no ratcheting is possible, even for arbitrarily large bending loads, despite the presence of the axial pressure load. The reason is that the case with a primary axial membrane stress exactly equal to half the primary hoop membrane stress is equipoised between tensile and compressive axial ratcheting, and hence does not ratchet at all. This remarkable result appears to have escaped previous attention.

Investigation of Crack Generation in a Notch during Diameter-Enlargement Working Method Processing

We have proposed a new cold processing method to enlarge the diameter of a short section of a metal shaft using a combination of a cyclic bending load and an axial compressive load that is lower than the yield stress of the sample material. We call this cold processing method the diameter-enlargement working method, and refer to the enlarged section of the processed shaft as the diameter-enlargement section. The processing method produces large plastic deformation, and its key features are as follows: the diameter-enlargement deformation progresses easily under a low axial compressive load at room temperature and the processed part exhibits little temperature increase. However, a crack is generated in the notch near the diameter-enlargement section during processing, and the cause is not yet clearly understood. Therefore, we conducted processing experiments to clarify the crack generation conditions, and simulated the working process using the finite element method to investigate the behaviors of stress and strain during processing. Furthermore, we calculated the low-cycle fatigue damage in the processed shaft using the Manson–Coffin expression. This study clarifies the mechanism of crack generation during processing and evaluates the fatigue strength of the processed part.

Structural Response of a Multi-Strand Stay Cable to Cyclic Bending Load

To date, only a few full-scale experimental investigations of the fatigue behavior of stay cables under axial loading and transverse displacements have been conducted. Modern cable anchorage systems often feature technologies that reduce local bending effects significantly and, therefore, interwire friction (fretting) may often constitute the main mechanism of the fatigue life reduction. However, the influence of transverse deformations and interwire movements at the critical locations on bending fatigue has not been measured and described in detail.The objective of the full-scale experimental investigation described herein was to study the structural response of a multi-strand stay cable to cyclic bending and to establish a correlation between the reversed cyclic flexural behavior of a single strand and that of a multi-strand stay cable. For this purpose, a digital image correlation (DIC) technique was employed as a tool for quantifying the deformations occurring on the surface of the strand and between wires. Moreover, the relationship between the transversal stiffness and the variation in tensile force of a single strand and that of a multi-strand specimen was studied.The novel application of the DIC technique for the measurement of local cable deformations provided valuable information on the internal state of displacement of the multi-strand stay cable under bending and resulted in the gaining of significant insight into the deformation and cyclic bending behavior of stay cable assemblies under axial and transverse loading.

Flexural fatigue behavior of 2D cross-ply carbon/carbon composites at room temperature

The primary goal of this research was to study the flexural fatigue behavior and enhancement of residual strength in 2D cross-ply carbon-fiber-reinforced carbon (C/C) composites. Flexural fatigue behavior was simultaneously examined under two stress levels (70% and 65%) by electrical resistance change (ERC) method. Residual strength and fracture behavior of C/C composites were investigated as well. The results show that shapes of electrical resistance change rate–fatigue cycle curves could reflect applied stress levels and damage types during cyclic bending load. The enhancement of residual strength only occurs at a stress level lower than the fatigue limit, and the property of enhancement decreases with the increase of fatigue cycles. In addition, three-point bending fracture in both unfatigued and fatigued specimens is of delamination mode and cyclic loading cannot enhance both bending strength and plasticity at the same time.

The bending fatigue comparison between 3D braided rectangular composites and T-beam composites

This paper reports the experimental comparison of quasi-static three-point bending and bending fatigue damage behavior between 3D braided rectangular composites (3DBRC) and 3D braided T-beam composites (3DBTC). The stress-deflection curves and failure modes were presented to compare the mechanical properties under quasi-static bending load condition. It was found that the load-carrying capacity of the 3DBTC was much better than that of the 3DBRC, up more than 19 percent. In addition, the S-N curves were employed to illustrate the comparison of fatigue life. The fatigue resistance performance of the 3DBRC was much better than that of 3DBTC at the same stress levels. The increase of of cycle numbers to failure under stress levels of 80 %, 70 %, 60 % and 50 % were 789.6 %, 395.7 %, 48.5 % and 132 %, respectively. The curves of damage indices vs. % of life (n/N×100 %) were given to characterize the three-stage accumulative fatigue failure process for both types of composites. Furthermore, the ultimate failure morphologies were presented to indicate the structural damage modes of the composites under the three-point bending cyclic loading condition.

Interfacial Behavior of RC Beams Strengthened with FRP under Fatigue Loading

The paper aims to study interfacial behavior of reinforced concrete (RC) beams strengthened with fiber reinforced polymer (FRP) under fatigue loading. A series of fatigue tests were conducted in the laboratory. The infrared detection technology was used to record the FRP-concrete interfacial fatigue behaviors. The results demonstrated that the fatigue development by RC beams strengthened with FRP was divided into three typical stages: rapid increase, stable-change, and uncontrollable propagation. The period of stable-change accounted for more than 99% of the fatigue lifetime. The average bending stiffness and the average FRP fatigue strain of the stable-change phase obtained under certain fatigue load were close to those obtained under the same monotonic load. The FRP threshold strain of the fatigue failure was about 37% of that of the monotonic load. The infrared thermograms showed that the interfacial fatigue debonding originated from the midspan and expanded to one end of the strengthened beam, which led to the final failure. The FRP fatigue strain and interfacial damage stayed stable in most of the fatigue lifetime, indicating good interfacial anti-fatigue properties. Based on the experimental data obtained in this study, a double logarithm formula was proposed to predict the fatigue life of RC beams strengthened with FRP under cyclic bending loads.

Ratcheting behavior of pressurized Z2CND18.12N stainless steel pipe under different control modes

With a quasi-three point bending apparatus, ratcheting deformation is studied experimentally on a pressurized austenitic stainless steel Z2CND18.12N pipe under bending load and vertical displacement control, respectively. The characteristic of ratcheting behavior of straight pipe under both control methods is achieved and compared. The cyclic bending loading and internal pressure influence ratcheting behavior of pressurized straight pipe significantly under loading control and the ratcheting characteristics are also highly associated with the cyclic displacement and internal pressure under displacement control. They all affect not only the saturation of the ratcheting strain but the ratcheting strain rate. In addition, ratcheting simulation is performed by elastic.plastic finite element analysis with ANSYS in which the bilinear model, Chaboche model, Ohno.Wang model and modified Ohno-Wang model are applied. By comparison with the experimental data, it is found that the CJK model gives reasonable simulation. Ratcheting boundaries under two control modes are almost same.

In vitro evaluation of pedicle screw loosening mechanism: a preliminary study on animal model

Materials and methods Eighteen vertebrae were harvested from the lumbar section of six porcine spines ranged from L1 to L3. All vertebrae were instrumented on both pedicles using pedicle screws. Quantitative CT scans were used before instrumentation in order to assess the bone density of each vertebra. Cyclic bending (toggling) load of ± 1mm displacement at 3Hz for 5000 cycles were assigned in two directions including craniocaudal (CC) and transverse (TR) toggling. Twelve instrumented pedicles were selected for CC toggling, twelve pedicles for TR toggling and twelve pedicles were not toggled (NT). All toggled and non-toggled screws were then pulled out at a displacement speed of 5mm/min in longitudinal direction. The peak pullout force and stiffness were computed from the load-displacement curves. Analyses of variance (ANOVA) were performed to investigate the effects of toggling methods and vertebral levels on the pullout force and stiffness.

Prediction of fatigue failure of coronary stent under cyclic bend loading using finite element analysis

Prediction of fatigue failure of coronary stent under cyclic bend loading using finite element analysis

Rotary Fatigue Testing Machine to Determine the Fatigue Life of NiTi alloy Wires and Endondontic Files

Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (MemryTM and EuroflexTM). The tests were done on a versatile fully automatic rotary bending testing machine. The formulation is also presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure.

Bending ratcheting behavior of pressurized straight Z2CND18.12N stainless steel pipe

The ratcheting effect greatly challenges the design of piping components. With the assistance of the quasi-three point bending apparatus, ratcheting and the ratcheting boundary of pressurized straight Z2CND18.12N stainless steel pipe under bending loading and vertical displacement control were studied experimentally. The characteristics of progressive inelastic deformation in axial and hoop directions of the Z2CND18.12N stainless steel pipes were investigated. The experiment results show that the ratcheting strain occurs mainly in the hoop direction while there is less ratcheting strain in the axial direction. The characteristics of the bending ratcheting behavior of the pressure pipes were derived and compared under load control and displacement control, respectively. The results show that the cyclic bending loading and the internal pressure affect the ratcheting behavior of the pressurized straight pipe significantly under load control. In the meantime, the ratcheting characteristics are also highly associated with the cyclic displacement and the internal pressure under displacement control. All these factors affect not only the saturation of the ratcheting strain but the ratcheting strain rate. A series of multi-step bending ratcheting experiments were conducted under both control modes. It was found that the hardening effect of Z2CND18.12N stainless steel pipe under previous cyclic loadings no matter with high or low displacement amplitudes is significant, and the prior loading histories greatly retard the ratcheting strain and its rate under subsequent loadings. Finally, the ratcheting boundaries of the pressurized straight Z2CND18.12N stainless steel pipe were determined and compared based on KTA/ASME, RCC-MR and the experimental results.

Effect of Cyclic Loading on the Residual Tensile Strength of Steel Fiber–Reinforced High-Strength Concrete

AbstractThis paper evaluates variations in the residual tensile strength of prismatic specimens made of steel fiber–reinforced high-strength concrete (SFRHSC) attributable to flexural cyclic loading. All specimens were previously subjected to a three-point static bending load test until the first flexural crack appeared. In these cases, no notched specimens were used. Next, the specimens were subjected to a preset number of three-point cyclic bending loads, without fatigue failure in any case. Finally, the specimens were submitted to a three-point static load test until failure. This test procedure shows a progressive decrease in the residual tensile strength with the number of cycles. A new definition of damage is introduced: relative variation of the tensile strength with the number of cycles. A correlation is identified between conventional damage and this new definition of damage.

Fatigue testing of CFRP in the Very High Cycle Fatigue (VHCF) regime at ultrasonic frequencies

Carbon fiber reinforced polymers (CFRP) are more and more used for high performance applications such as aircraft structures. These structural components are subjected with more than 108 cycles during their lifetime. At present the fatigue behavior of CFRP and the corresponding failure mechanisms are not sufficiently investigated in the VHCF regime. To obtain comprehensive knowledge about the fatigue behavior as well as the failure mechanisms of CFRP in the Very High Cycle Fatigue (VHCF) regime, a new Ultrasonic Testing Facility (UTF) for cyclic three point bending of CFRP at 20 kHz has been developed at the Institute of Materials Science and Engineering (WKK), University of Kaiserslautern. This UTF with combined online nondestructive testing via laser vibrometry and IR-thermography enables VHCF experiments up to 109 cycles in twelve days. All experiments are carried out at a stress ratio between 0.29 and 0.49. To keep the specimen temperature below the glass transition temperature of the CFRP the cyclic bending load is divided in pulse pause sequences. The specimen material used in this investigation is a commercially available carbon fiber fabric reinforced polyphenylensulfide (CF-PPS). For the accurate calibration of the loads as well as for the visualization of the strain distribution and the oscillation mode, measurements with 3D-scanning-laser-vibrometry have been carried out. Furthermore light optical micrographs plus SEM investigations have been performed upon completion and in defined interceptions of the experiments. Based on the results of the microscopic investigations the failure mechanisms of CF-PPS for the VHCF are described.